I have a live web application(.NET), in which I have a facility for making transactions. When more no. of users make any transactions at the same time, duplicate/same transaction ID gets generated for all the users who're all making transactions. Is there any way to avoid creating same ID ? I tried the following solutions but nothing helps.
1.Mutex
2.Table lock(SQL)
3.Generating Transaction ID at the time of inserting into the table.
Use a database to generate the ID. Some possible examples:
SQL Server has an auto increment feature. Oracle has a sequence feature
This will ensure your ID to be unique.
You mention SQL, so you have a database accessible.
Option 1
SQL Server already has an auto-increment feature that is both guaranteed to generate a unique ID and is efficient.
If you want to "add letters to that ID", the simplest solution is to add a separate varchar field that contains those letters, and then format them for display as a single number in your application (either by using a SQL query to do so or string.Format.
ID | IDText Application Display
------------------ -----------------------
1 | MyLabel 1-MyLabel
2 | MyLabel 2-MyLabel
3 | FooBar 3-FooBar
4 | SomeText 4-SomeText
So, in the above, ID would be an auto-increment int field and IDText would be a varchar (or char if you want a fixed length).
As you can see, even if the same string such as "MyLabel" were added at the same time, you would get a different ID to append to it so you in effect have a unique ID.
Option 2
Use a stored procedure to contain the following logic in a single transaction with BEGIN TRANSACTION/END TRANSACTION:
Query for the highest ID
Add 1 to that ID
Create a new record with the new ID
Return that ID
The transaction will guarantee that the number cannot be duplicated. This is less efficient for creating IDs, but more efficient at looking them up than the 2-column approach.
Note 1: I REPHRASED THE QUESTION. It now consists of Suppliers and Orders, instead of Cars and Parts.
Note 2: THIS PROBLEM IS HYPOTHETICAL.
My goal is to understand how to create object counters.
For regulatory requirements, I need TO SEQUENTIALLY NUMBER EACH Order for each of the suppliers.
I'm Using 'Entity Framework` with Sql Server.
In my hypothetical example, I have a Supplier class and an Order class.
Each supplier has Orders. Each order has a product and a quantity. Meaning, it states which product was ordered from the supplier and how many of it.
I need to be able to create counters, like an auto incremented number, to count the orders FOR EACH supplier.
For regulatory reasons, each supplier must sequentially number its orders, in the order of creation, and using an integer only.
When we examine an Order, We should know by its OrderCountForSupplier column, what was its order of creation (a DateTime / TimeStamp column is insufficient by the regulatory authorities. They require such a counter).
For simplicity of this question, an order cannot be deleted (it's status can change, but it cannot be deleted).
It's very important for me to have a solution which includes the technical/programming way, not only theoretic way.
I've made a diagram in order to explain my problem in the most clear way possible:
I have a way that might work, and would be glad to hear feedback.
I'm thinking of an external table/tables, to hold the counters. Something like:
Supplier Order Counters Table
| SupplierId | OrderCountForSupplier
------------------------
| 54654 | 3
| 78787 | 2
| 99666 | 4
Would I need a trigger in order to increment the OrderCountForSupplier counter on each insertion, for each supplier?
If not - how can this incremental be done in a safe way ? (without for example, two processes in a race condition to get the next counter and increment it, which could eventually result in a duplicate Order Count).
And another note:
Can this be done Entity Framework wise? if not - a Sql Server solution will be respected.
First answer, the example in the question has changed after it was written.
You say that is it OK to have gaps in the Part IDs, because "some parts might be deleted along the way".
So, what's the difference between your example:
Car PartID
54654 1
54654 2
54654 3
78787 1
78787 2
99666 1
99666 2
99666 5
99666 7
And this variant:
Car PartID
54654 1
54654 2
54654 3
78787 4
78787 5
99666 6
99666 7
99666 8
99666 9
In the second variant each part has some ID that is unique for each car (it is also globally unique as well, but it doesn't matter). In the second variant PartID specifies the order in which parts were inserted into the table, same as in the first variant.
So, I'd use a simple IDENTITY column:
Parts
PartID int IDENTITY NOT NULL (PRIMARY KEY)
CarLicenseNum int NOT NULL (FOREIGN KEY)
PartName varchar(255)
Update for Supplier-Order example
The most important bit in the updated question is "regulatory reasons". It answers the question why would you want to do such unnatural thing. "Regulatory" and efficiency are often opposite.
Essentially, it means that you have to use serializable transaction isolation level when inserting a new row and calculating the next number in the sequence. It will hurt concurrency/throughput, but it will guarantee consistency and "be safe" in multi-user environment.
I don't know how to do it in Entity Framework, it should be possible. But, again, for "regulatory reasons" I'd put this logic in the stored procedure in the DB and make sure that ordinary users don't have write access to the Orders table directly, but have rights only to execute this dedicated stored procedure. You can replicate the logic of this stored procedure in the EF code, but the database itself will be open to changes done through other applications, which may not follow the regulatory requirements.
You can implement it using the separate table, which stores the latest sequence number for each supplier, or you can read the last maximum sequence number on the fly. If each supplier has only few orders, then this separate table with latest values of counters would be comparable to Orders table and you would not gain much. In any case, having a proper index is the key. Getting the latest counter value would be one seek in the index.
Here is an example of stored procedure without using an extra table.
Make sure that Orders table has unique index on (SupplierId, OrderCountForSupplier). In fact, you must have this index even if you are using an extra table to enforce the constraint.
CREATE PROCEDURE [dbo].[AddOrder]
#ParamSupplierID int,
#ParamProductSerial varchar(10),
#ParamQuantity int,
#NewOrderID int OUTPUT
AS
BEGIN
SET NOCOUNT ON;
SET XACT_ABORT ON;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
BEGIN TRY
DECLARE #VarMaxCounter int;
SELECT TOP(1) #VarMaxCounter = OrderCountForSupplier
FROM dbo.Orders
WHERE SupplierID = #ParamSupplierID
ORDER BY OrderCountForSupplier DESC;
SET #VarMaxCounter = ISNULL(#VarMaxCounter, 0) + 1;
INSERT INTO dbo.Orders
(SupplierID
,OrderCountForSupplier
,ProductSerial
,Quantity)
VALUES
(#ParamSupplierID
,#VarMaxCounter
,#ParamProductSerial
,#ParamQuantity);
SET #NewOrderID = SCOPE_IDENTITY();
COMMIT TRANSACTION;
END TRY
BEGIN CATCH
-- TODO: handle the error
SET #NewOrderID = 0;
ROLLBACK TRANSACTION;
END CATCH;
END
GO
After investigating some possible approaches (see links at the bottom), I've came out with a very basic solution, with the help of #Vladimir Baranov.
I've ruled out using SqlServer triggers / Stored Procedures. They seemed hard to implement in conjunction with Entity Framework, and they seem to me like an Over-Kill in this scenario.
I've also ruled out the Optimistic Concurrency approach (using a concurrency token), because in this scenario, the counters cannot be updated simultaneously. They only get updated after a successful insertion to the orders table.
My orders table looks like that. I've added a unique constraint on the OrderId, SupplierId and OrderCountForSupplier trio, so insertion of the same order count for a supplier would fail.
I've indeed used a counters table, from which I can take the latest counter - for each of the suppliers.
Supplier Order Counters Table
| SupplierId | OrderCountForSupplier
------------------------
| 54654 | 3
| 78787 | 2
| 99666 | 4
These are the steps:
Get the current supplier orders counter.
Try insert a new order for the supplier, using the current counter + 1.
If the insertion goes ok => Increase the orders counter for this supplier, on the supplier counters table.
If insertion goes wrong, and we get an error stating the has been a violation of the constraint (same order count, which already exists):
Try 2 more times to get the current counter, and try inserting the order again.
The Code:
public class SupplierRepository
{
private MyContext _context;
private Supplier _supplier;
public SupplierRepository(int supplierId)
{
_context = new MyContext();
_supplier = context.Suppliers.Single(x => x.SupplierId == supplierId);
}
// Retrieve the latest counter for a supplier
public SupplierCounter GetCounter()
{
var counterEntity = _context.SupplierCounters.Single(x => x.SupplierId == _supplier.SupplierId);
return counterEntity;
}
// Adding a supplier
public void AddSupplier(Order order)
{
int retries = 3;
while (retries > 0)
{
SupplierCounter currentCounter = GetCounter();
try
{
// Set the current counter into the order object
_order.OrderCountForSupplier = currentCounter.OrderCountForSupplier;
_context.Add(order);
// Success! update the counter (+1) and then break out of the while loop.
currentCounter.OrderCountForSupplier += 1;
// I'M CALLING `SAVECHANGES` AFTER ADDING AN ORDER AND INCREASING THE COUNTER, SO THEY WOULD BE IN THE SAME TRANSACTION.
// THIS WOULD PREVENT A SCENARIO WHERE THE ORDER IS ADDED AND THE COUNTER IS NOT INCREMENTED.
_context.SaveChanges();
break;
}
catch (SqlException ex)
{
if (ex.Number == 2627) // Violating unique constraint
{
--retries;
}
}
}
}
}
Some useful links:
SQL Server Unique Composite Key of Two Field With Second Field Auto-Increment
Atomic Increment with Entity Framework
how to inc/dec multi user safe in entity framework 5
This is not a real world example. That's why you are struggling. For an example, A real world parts entity is lot more complicated than that. A real world part will have a ManufacturerId (BMW, Audi etc), PartNumber(B4-773284-YT), VehicleModelId (AUDI A4 etc), Description, ManufacturerYear so on and so forth. Usually when it comes to parts entities, we use a concatanated primary key on ManufacturerId and PartNumber.
Same with your car table. It's not a real world example too. Car entity should have a VIN number, which is unique. When you say each part is specific, you are not talking about Part entity. You are talking about PartInventory entity. PartInventory has a unique serial number (barcode) for each part. So every single part can be identified uniquely. When you attach a part to a vehicle, you are not just attaching a Part, you are actually attaching a PartInventory item, which is recognizable by a unique serial number.
Once the partInventory item is attached to a vehicle, it becomes a fitted part item of the vehicle. Which means the part gets transferred to VehicleParts table.
Unfortunately I see a lot of gaps in your vehicle industry domain knowledge. We develop systems to address real world problems. When you try to address hypothetical problems, you run in to this kind of issues. That leads to wasting lot of other peoples time who are trying to help you out.
First things first: it is not OK to change your question entirely! Delete this question and create a new one. Having said that ...
Answer of the current question:
Answers to hypothetical questions are just oppinion based and/or too broad (there is actually a flag for this - Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.)!
My answer to the current question is: I do not see any benefit (or advantage or use) of the OrderCountForSupplier in the database! Creating such counter in the database makes adding and maintenance (in a multi-threaded environment) very complicated and error-prone.
I think the problem can be solved more easily with the help of EF (move the creation of the counters in the code) and a different design of the database:
in order to allow concurrent adding of Orders, create two columns - a GUID as the Order-PrimaryKey and a CreationDate of type DateTime. Filling those two columns from multiple threads is not a problem
when retrieving all Orders for a specific SupplierId, sort the result list in ascending order by CreationDate
when iterating over the result list using (for example) a for-loop, then the counter is the desired sequential counter
as an alternative to the EF solution, the creation of the sequential counter can stay in SQL - create a view or stored procedure for the Order items and use ROW_NUMBER to create the artificial sequential count, after grouping the items over SupplierId and sorting on CreationDate
Reading the database from multiple threads (and creating the counter in every thread) is again not a problem any more.
Answer of the first question:
You are almost there. You need to normalize your data model a little bit more. This is a common scenario in which you want to minimize redundancy of the data and at the same time still maintain a meaningful relation (without the use of triggers).
One possible solution would be to create a Car_has_Part-Table in order to represent the relation between a Car and a Part entity:
| Car_has_Part |
----------------
| PartId |
| CarId |
The primary key of the Car_has_Part table is a composite primary key consisting of CarId + PartId which is unique and at the same time you avoid data duplication.
In your example in the Parts table the Doors part is repeated for every Car. Using this intermediate table the data is not duplicated and you have a proper relation.
Your new data model could look like this:
| Car | | Car_has_Part | | Part |
------- ---------------- ----------
|CarId | | PartId | | PartId |
|Model | | | | Descr |
| etc. | | CarId | | etc. |
This model allow resp. covers the specified requirements:
I need to be able to create a counter, like an auto incremented
number, to count the parts for each car. Car 1, could have parts 1, 2,
3... and Car 2 would also have parts 1, 2, 5, 7... (some parts might be deleted along the way).
Select all PartId's from the Car_has_Part table over CarId.
Each part HAS to be counted separately for its related car. That's the
base requirement.
Same as above (without data duplication like in your example). Adding resp. removing a relation or modifying a part name has also become easier - you need to update only one row in the Parts table and the change is reflected for every car.
About the triggers question - you can only create a trigger with EF (using code first approach). Regarding execution - triggers are always executed in the database and EF can't control trigger execution (you can certainly enable/disable trigger using raw SQL queries, but if I understand your question correctly this is not what you want).
Table
ID | Name | NricNo
1 | Joshua | S1234567A
2 | Joshua | S1234567A
I have a problem for this table. ID is the primary key of this table, but I need to make Name,NricNo to be unique too. It means that table cannot have duplicate value of Name and NricNo. I am doing this in visual studio 2010, help needed . Thanks :)
You can simply alter your table with this query.
ALTER TABLE
ADD CONSTRAINT UNIQUE ();
GO
--Example :
ALTER TABLE MyTable
ADD CONSTRAINT AK_MyTable UNIQUE (Name, NricNo);
GO
This all depends on how you're doing your database interactions. If you're using Entity Framework, as long as you're not creating a new object with the same values, you should be fine.
Either way, you could help yourself by adding an ID field to your data object. If you're using ADO.NET, then check the ID in your save method. If it's not equal to a default value (you should default it to -1 to be safe), then Update, otherwise Insert. Have your saving in a try/catch block because with the UNIQUE constraint on the table, if you try to insert a record that conflicts with the UNIQUE key, then it will throw an error. If you use Entity Framework, most of that would be handled for you, but you should still have the try/catch in your save method.
Is there any way to alter the underlying database using EF using Code First approach?
I have 2 tables which have a static model:
Users and Info1.
I also have another table which Ill call info2.
I would like to be able to add and remove columns from Info2 from the admin section of my website.
My goal is to have a website which can dynamically be altered as you go, adding and removing fields as the user likes, without the user having to know anything about coding.
I've considered using a separate database outside of the one specified in the model of my MVC3 project and do straight SQL requests to that instead.
This could also be accomplished by having a table with the dynamically created fields, and another with the data, but this gets messy fast.
Has anyone done anything like this? Is it a bad idea?
I'd recommend not trying to expand the table horizontally, that's an operation that you should make a conscious decision to have.
Instead, I'd recommend that you store the values as name/value pairs. You can have tables that have specific types (let's say you needed an integer value paired with a key), and then you would select those into a dictionary for the user.
You'd also have a table which has the keys, if you are concerned about replicating key values.
For example, you'd have a UserDefinedKey table
UserDefinedKeyId (int, PK) Key (varchar(?))
-------------------------- ----------------
1 'My Website'
2 'My favorite color'
Then you would have a UserDefinedString table (for string values)
UserDefinedStringId UserId UserDefinedKeyId Value
(int, PK) (int, FK) (int, FK) (varchar(max))
------------------- --------- ---------------- --------------
1 1 1 'http://stackoverflow.com'
2 1 2 'Blue'
3 2 2 'Red'
You'd probably want to place a unique index on the UserId and UserDefinedKeyId fields to prevent people from entering multiple values for the same key (if you want that, have a separate table without the unique constraint).
Then, when you want to add a value for users, you add it to the UserDefinedKey table, and then drive your logic off that table and the other tables which hold the values.
Another benefit of storing the values vertically is that you aren't wasting space for columns with values that aren't being used by all users.
For example, assuming you take the approach of modifying the table, for the attributes above, you would get:
UserId WebSite Color
------ ------- -----
1 http://stackoverflow.com Blue
2 (null) Red
Now let's say a third user comes along, and adds a Favorite Sports Team value, and they are the only one who uses it, the table then looks like:
UserId WebSite Color FavoriteSportsTeam
------ ------- ----- ------------------
1 http://stackoverflow.com Blue (null)
2 (null) Red (null)
3 (null) (null) Yankees
As the number of users and attributes grows, the amount of sparse data that you have will increase dramatically.
Now, assuming you are using SQL Server 2008, you could use sparse columns, if you don't, your table is going to get huge but not have much data.
Also, using sparse columns doesn't take away from the fact that it's pretty dirty to use data definition language (DDL) to change the schema on the fly.
Additionally, Entity Framework isn't going to be able to adapt it's object model to account for the new attributes; every time you have an attribute added, you will have to go and add the attribute to your object model, recompile, and redeploy.
With a vertical approach, it takes more work, granted, but it will be infinitely flexible, as well as utilize your database space more efficiently.
I'm working with a legacy table which I cannot change. The database have a settings table which consists of a lot of columns (one for each setting) and only one row:
Columns: Setting1 | Setting2 | Setting3 | etc...
----------+--------------+-----------+-------------
Row1: SomeValue | AnotherValue | LastValue | etc...
Now, this would be all fine if it wasn't for the fact that the table lacks a primary key. Obviously, the original developer didn't think it was necessary, as there's only one single row.
Is there any way of mapping this with Nhibernate? I've already implemented a SQL based solution, but I'd love to have the flexibility and simplicity gained with Nhibernate.
I'm fearing the worst but, any ideas?
You could map a view that adds a dummy PK column:
select 1 as SettingsId, Setting1, etc....
or load the object from a stored procedure.
Well, just pick any setting (column) as the Id. The only inconvenience is that if you need to change the setting that is the PK you'll have to delete and re-insert the row...